1. Field of the Invention
The invention relates to a method of attaching a component. The invention is preferably applied for attaching an integrated circuit (IC) or other component electrically by the Direct Chip Attachment (DCA) method to a circuit card, interposer or other substrate and for protecting the area remaining between the component and the substrate.
2. Description of the Related Art
A known technique is the attachment of semiconductor components to a circuit card underneath the component by means of column-like conducting parts that are formed on the pads. These parts are generally fabricated from a metal such as tin and the parts are called bumps. The component is mounted resting on these bumps. A known method is so-called Flip Chip Technology (FCT), according to which non-encapsulated semiconductor components are attached to the substrate with metal bumps by soldering or by melting the bumps. The bumps are formed by the following process stages: vacuum deposition of the pads, i.e. the attachment pads, using another metal such as tin or lead in combination with a subsequent soldering process. The bump can also be made of gold. With this technique, the bump pitch can be reduced to a minimum of about 200 .mu.m. The bump pitch is here taken to mean the distance between the centre points of adjacent bumps. Following the attachment, the free space remaining between the component and the circuit card is filled with an insulation using a special filling process in order to protect the bonding side of the component.
The problem with this technology is that the metal bumps and joints break easily, e.g. due to temperature variations. The shear stresses of the joint are also reduced by a mounting process in which the component is raised when the bumps have melted. In order to reduce the forces to which the joint is subjected and to protect, the space between the attached component and the substrate has to be filled with an insulating material. For this reason, space must be reserved around the component for properly apportioning the insulating material, that is, filling the space between the component and the substrate with a correct amount of the insulating material. The substrate must be heated so that the insulating material spreads out under the component. Mastery of the spreading of the insulating material and the simultaneous curing is a demanding process and, furthermore, the central area that is comprised of components attached using several concentric rows of bumps often remains unfilled. A further problem is that testing of the loose microcircuits causes holes in the bumps, thereby impairing the attachment. For FCT bumps, the integrated circuit must from the outset be especially designed or a special metal surface must be applied to its surface in order to make the bumps.
Also previously known from the article Characteristics and Potential Application of Polyimide-Core-Bump to Flip Chip, Takashi Nishimori et al., Proceedings of Electronic Components & Technology Conference (ECTC) 1995, pages 515-519, is a column-shaped bump having an elastic core. The completed bump with its surrounding area is shown in a cross-sectional view in FIG. 1. The bump 3 is made using a multistage method in which the mentioned core 3a is formed from photosensitive polyimide on top of the attachment pad 2 of component 1. An even layer of polyimide is spread with a spinner on the surface of the component at a speed of about 2000-5000 1/s rotations per second. Photosensitive polyimide is exposed to ultraviolet light at the middle of the attachment pad 2 at the point of the bump 3 which protrudes through the mask, so that the photosensitive polyimide remains on the attachment pad during the stage of removing the excess material, and the excess polyimide is also removed. The core 3a of the bump and the surrounding free area of the attachment pad 2 are metallized with a homogeneous layer 3b when the core 3a is formed in the above-mentioned manner on the attachment pad 2. The metal surface 3b is spread by the sputtering method in which a coating is formed on the surface of the component 1. Removal of the excess metal coating can be carried out with several alternative methods including etching, etching after plating or a liftoff process. The bump is bonded to the circuit card with solder or anisotropic glue. During bonding of the component, oxidized spots on the metal surface are removed most preferably with flux in the soldering process. The metallized bump is bonded both electrically and mechanically to the attachment pad, which is covered with circuit card solder in such a way that the fused solder rises up along the edges of the bump that has been pressed into the circuit card's attachment pad from the attachment pad of the circuit card towards the attachment pad of the component, surrounding the bump with solder. The dimensions of the bump are preferably 30 .mu.m high and 30 .mu.m wide. The bump pitch is 90 .mu.m at its smallest.
Previously known technique is also presented in the following patent applications: JP-A 8-213 400, Solder Bump, Formation of Solder Bump, and Solder Bump Forming Body, Toshiba Corp., 8.2.1995 and DE-A1-42 23 280, Schaltkreistrager-Baueinheit und Verfahren zu deren Herstellung, Sharp K.K., 15.7.1992.
The problem with this method is the weak or non-existent formation of a metal layer 3b on the vertical surface of core 3a of the bump in the sputtering stage. During bonding the solder spreads along the metal surface 3b and does not adhere to the bump at the place where there is not a sufficient metal layer 3b. A further problem is the brittleness and stresses of the intermediate metals formed between the core 3a and the metal layer 3b, since the brittleness and stresses can cause the metal layer 3b to work loose from the surface of the core 3a of the bump. Furthermore, there is a problem in that sufficient space must be arranged around the core 3a to provide contact of the metal surface 3b with the attachment pad 2. A further problem is that the metal layer 3b on the straight vertical surface is not flexible, which means that the thin metal layer 3b cracks easily when the stress caused by the flexing of the bump 3 is directed at the junctures of the horizontal and vertical surfaces of the metal layer 3b. The problem in methods where the excess metal layer is removed is that when a photoresist such as photosensitive polyimide is spread on them with a spinner after formation of the core 3a of the bump, there are formed behind the core 3a of the bump, in the direction of spreading of the photoresist, spots where only a little photoresist is applied or none at all. This leads to flaws in the pattern on the metal surface during the etching stage. Another problem is that the metal layer 3b of the bump becomes oxidized, which is due to the properties of the metal required in the method. Oxidization hampers electrical testing of the component 1 because a proper electrical connection cannot be made to the bumps 3 during testing.